Read coil status from slave using Modicon 984LL (concept)


Thread Starter


I am Using Redundent Modicon Quantum PLC with 984LL. I have used MSTR block to read 4x registers from slave devices. Concept manual says MSTR can be used for 4x registers only.

How can I read 0x, 1x & 3x registers from slave devices?

XMIT loadable is not available with my PLC.

software i am using : Concept v2.6sr4
The routing path is through BM85 (the slave device is connected to BM85 not directly to the PLC)
PLC: 140 CPU 534 14 (This PLC will support IEC but i am just modifying the existing 984LL)

Looking for a help.....

Don Anderson

To read 0x, 1x or 3x bits or registers they must be "packed" into a 4x registers before they are read. The normal way to do the packing is to use a BLKM (Block Move) instruction.

Unpacking the 0x or 1x bits at the receiving end can also be done utilizing a BLKM instruction.

email me if you need more information.
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Don Anderson's advice refers to the specific Modicon PLC instruction for moving bit data to registers, but generally applies to any Modbus-capable PLC. That presumes your slave devices are PLC's and not other Modbus-capable devices such as valves, instruments or remote I/O.

In the case of other devices, you won't have a move instruction to program, but my experience is that the device will mirror I/O and bit data to register addresses. Look through the address definition table in the manual and you'll see entries such as 40175-40180 equals 30001-30006 or 40192 equals 10001-10016.

In either case you'll have use the MSTR instruction in your PLC to read the data from 4x registers then move it to 0x coils or use the SENS instruction to get the individual bits.